Photoresist patterning is a key step in the formation of integrated circuits in semiconductor devices. A photoresist, hereafter referred to as resist, is typically spin coated on a substrate, baked to form a film, and patternwise exposed by employing an exposure tool and a mask that contains a device pattern. Radiation is transmitted through transparent regions of the mask to selectively expose portions of the resist layer. The resist layer is developed in a media such as an aqueous base solution to produce a resist pattern on the substrate. Each technology generation or node in the microelectronics industry is associated with a particular minimum feature size in the resist pattern. As technology advances have been continuous in recent years, the minimum feature size requirement has rapidly shifted from 250 nm to 180 nm and then to 130 nm. New products are now being developed for a sub-100 nm technology node.
Some of the more common features that are printed in resist layers are contact or via holes and trenches which have a variety of pitches. In FIG. 1, a resist layer 2 is patterned on a substrate 1. In one region of the pattern, a pitch P1 is equal to the space width W1 of a feature such as hole 3a and the distance D1 separating hole 3a from an adjacent hole 3b. Another region of the pattern has a pitch P2 consisting of a space width W1 in a hole 3c and a distance D2 between hole 3c and an adjacent hole 3d. The ratio D1/D2 may vary from slightly more than 1 to a number as high as 10 or more. One of the problems associated with a typical patterning process is that space width W1 is dependent on pattern density. For example, space width W1 in an opening like hole 3c that is part of a dense array is printed at a different size than space width W1 for a semi-isolated hole 3a or an isolated hole in the same pattern even though the space width on the mask used to print the pattern is the same for all of the holes 3a–3d. As a result, optical proximity corrections (OPC) are required on the mask design that will enable the lithography process to print dense and isolated holes with equal space widths W1. OPC can be cumbersome to generate and a period of one or two months may be necessary before a new mask with OPC corrections is available. It is desirable to have an alternative method in which the pattern density in a resist pattern is adjusted so that holes 3a–3d are all printed with the same space width W1.
The minimum resolution that can be achieved in a printed pattern is defined by the equation R=kλ/NA where R is the minimum feature size that can be resolved, k is a constant, λ is the exposure wavelength, and NA is the numerical aperture of the exposure tool. While exposure tools having mercury lamps that emit g-line (436 nm) or i-line (365 nm) radiation have been widely used in the industry, the trend in newer technologies is to move to shorter wavelengths such as 248 nm from KrF excimer lasers or 193 nm from ArF excimer lasers to achieve smaller feature sizes approaching 100 nm. In the near future, 157 nm radiation from F2 lasers and 13 to 14 nm wavelengths from extreme ultraviolet radiation (EUV) sources will be available for printing sub-100 nm features. Projection electron beam (e-beam) tools are also being developed for sub-100 nm applications.
A method of forming smaller contact holes by a double exposure process described in U.S. Pat. No. 5,573,634 may be applied to any UV wavelength since it lowers the amount of diffracted light from a single exposure. The technique avoids exposing adjacent holes on a single mask which produces a significant background intensity between the holes in the aerial image that reaches the resist layer.
Commercial resist compositions are available in two general types that are referred to as positive tone and negative tone formulations. In positive tone or positive resist, exposed regions become soluble in a developer solution that is typically an aqueous base. Unexposed regions in the film stay insoluble in the developer and remain on the substrate. For negative resists, exposed regions become insoluble in a developer while the unexposed regions remain soluble and are washed away. The resist solution is spin coated on a substrate and baked to form a film thickness that may vary from about 0.2 microns to several microns. As a general rule, the thickness is about 3 or 4 times the size of the minimum space width or line width. Therefore, to print a 100 nm contact hole, a 300 to 400 nm thick film is typically applied in order to have a patterning process latitude that is manufacturable.
Most state of the art positive and negative resists operate by a chemical amplification mechanism in which a photosensitive component absorbs energy from the exposing radiation and generates a strong acid. One acid molecule is capable of removing many polymer protecting groups in a positive resist mechanism or initiating several crosslinking reactions in a negative resist mechanism. A post-exposure bake is usually required to drive the reaction to completion within a few minutes so that the process is compatible with a high throughput manufacturing scheme. Chemically amplified (CA) resists are especially useful with Deep UV (248 nm) radiation or with sub-200 nm exposure wavelengths. Another important feature of a CA resist is that in addition to a polymer, solvent, and photoacid generator component, the CA resist also contains a quencher which is usually a base such as an amine that controls acid diffusion in the exposed film and acts as an acid scavenger in the resist solution.
The negative resist imaging process may involve a crosslinking mechanism or a polarity change to render the exposed regions insoluble in developer. Crosslinking occurs when a photo generated acid catalyzes bond formation between two polymer chains or between a polymer and an additive containing reactive groups. Depending on the molecular weight (MW) of the original polymers, a few crosslinks are all that might be needed to convert a soluble polymer into an insoluble network of polymers. This solubility difference is the basis for forming a pattern in an exposed negative tone film.
Traditionally, resists have been formulated in organic solvents, but recently water based formulations that are more environmentally compatible have been developed. U.S. Pat. No. 5,017,461 describes a water soluble negative tone composition based on a polyvinyl alcohol (PVA) and an acid generator that is a diazonium salt. An hydroxyl group on the polymer reacts with the diazonium salt to form an ether and liberate nitrogen and HCl. When the film is heated, HCl induces the polymer to lose a molecule of water and form an alkene that is insoluble in water developer. This is an example of a negative resist based on a polarity change.
Another water soluble negative resist that does not rely on a crosslinking mechanism is provided in U.S. Pat. No. 5,998,092. A photoacid reacts with an acetal group on a polymer side chain to produce a B-keto acid that loses CO2 to form a polymer which is insoluble in aqueous base developer. This composition is especially useful in avoiding swelling in aqueous developer.
A water soluble resist that is compatible with a crosslinking mechanism is described in U.S. Pat. No. 5,948,592 in which a calcium salt of an organic acid is added to an aqueous form of casein, a photosensitive material, and optionally, a crosslinker. An acetate, lactate, or formate salt is used to improve photosensitivity, resolution, and etch resistance in a resist pattern that may be hardened by baking from 150° C. to 300° C.
Individual components of negative resists have been developed that possess water solubility as an added property. For example, a water soluble sugar is claimed as an improved crosslinker in related U.S. Pat. Nos. 5,532,113 and 5,536,616. This crosslinker is used in combination with a p-hydroxystyrene polymer and a triphenylsulfonium salt that are not soluble in water and have an optical absorbance that is most suitable for 248 nm exposures. The pattern is developed in aqueous base. In U.S. Pat. No. 5,648,196, a water soluble photoacid generator (PAG) is described and is formulated with a p-hydroxystyrene polymer and a water soluble sugar. Either water or aqueous base developer is acceptable. The PAG is preferably a dimethylarylsulfonium salt wherein the aryl group has one or more hydroxy substituents.
Still another crosslinking formulation is provided in U.S. Pat. No. 5,858,620 in which a water soluble polymer and crosslinker are coated on a patterned layer containing acid that has a hole with a space width of about 400 nm. The patterned layer is either baked at 150° C. to cause acid diffusion that induces crosslinking in the water soluble layer or the patterned resist is exposed and baked to drive acid into the top layer. In either case, a crosslinked coating is formed on the patterned resist that effectively shrinks the space width to about 300 nm. In related art, U.S. Pat. No. 6,319,853 describes a crosslinking mechanism to shrink a 200 nm space to a 110 nm space width. However, the crosslinking layer does not contain a quencher and the extent of acid diffusion is determined by only the bake temperature and time which may be difficult to reproduce uniformly across a wafer.
Therefore, a improved method that offers a higher degree of control in shrinking space widths which is desirable for new technologies involving hole patterns with space widths approaching 130 nm or smaller is needed. A process that is able to shrink space widths of holes in addition to adjusting pattern density is especially appealing to manufacturing since it provides more flexibility in the overall scheme of fabricating semiconductor devices.